Getting a nice camera is just the first step in taking great photos—you also have to learn how to use it. Shooting on auto will only take you so far. Shutter speed, aperture, and ISO may sound like intimidating photographer terms, but they’re quite simple—and crucial to getting great photos.

It’s All About Exposure

Deep inside every digital camera is a photographic sensor that records the images you shoot. When you take a picture, the shutter that normally covers the sensor opens, and the light that’s coming in through the lens falls on the sensor where it gets converted into digital data.

A photo will look very different depending on how much light reaches the sensor. If only a little bit of light hits the sensor, the image will be much darker than one where light came flooding in.

For any scene, there will be an ideal amount of light to let in. If you let too little light hit the sensor, the scene will look too dark; if you let in too much, it will look too bright. You can see an example of what that looks like in the photo below.

There’s a thin line between jargon and legitimate technical terms, but with photography there are some words you need to know. Every time you take a photo, you’re “making an exposure”. If the settings are right, it will be a “good exposure”. If the photo is too dark, it’s “underexposed”. If it’s too bright, it’s “overexposed”.

When it comes to controlling how much light reaches the sensor—aka controlling your exposure—you have two main options: change how long the shutter stays open (we call that the “shutter speed”) or change how big the opening in the lens that lets light through is (that’s the “aperture”). The longer the shutter speed or the wider the aperture, the more light gets let through.

If you’re shooting with “natural light” (meaning you’re not using any flashes), the amount of light available in each scene is fixed. To make a good exposure, you need to use some combination of shutter speed and aperture that lets the right amount of light hit the sensor. In a dark room, you don’t have much light to work with, so you’ll want to use the longest shutter speed and widest aperture you can. On a bright sunny day, however, it’s really easy to overexpose your photos, so you need to limit how much light reaches the sensor. In those cases, you won’t be able to use wide apertures and long shutter speeds, or at least not together.

This would all be easy, except shutter speed and aperture have other effects on your photos, too. Feeling overwhelmed yet? Don’t worry, we’ll take you through the basics. Let’s start with shutter speed.

How Shutter Speed Affects Your Photos

Shutter speed, again, refers to how long the shutter stays open when you take a photo. Most cameras can handle shutter speeds of around 1/4000th of a second up to 30 seconds. The shutter speed—you might also see it called the “exposure length”—affects exposure as described in the previous section, while also determining how movement is recorded in your pictures.

I took the picture below with a shutter speed of 1/2000th of a second. There’s a storm brewing tonight in Ireland, so it’s really windy out. Looking at this picture, though you wouldn’t know it. The leaves are frozen in place.

This image was taken a few moments later, with a shutter speed of 1/15th of a second. Look at how the leaves are now blurry in some places. That’s because during that 1/15th of a second the shutter was open, the leaves moved.

If you’re using a camera without a tripod, there’s a limit on how slow a shutter speed you can use. If it’s any less than about 1/100th of a second, there’ll be some motion blur just from your hands pressing the shutter button.

How Aperture Affects Your Photos

Aperture is the size of the opening that light passes through in the lens. It’s measured in “f-stops”. Most lenses have a maximum aperture of between f/1.8 and f/5.6, and a minimum aperture of f/22.

Although it’s not important to remember, an f-stop is the ratio between the “focal length” of the lens and the aperture. If a lens with a 50mm focal length is set to an f-stop of f/2.0, the aperture is 25mm wide—you divide the focal length (f) by the number underneath.

That means—and this is the part you need to remember—the lower the f-stop, the wider the aperture is open, and thus the more light that’s let in.

Aperture affects the exposure of your photo, but it also controls the “depth of field” (how much of the photo is in focus). The wider the aperture is, the thinner the area of the image that will be in focus. If you look at the image below, which I shot with an aperture of f/1.8, only the model’s face is actually in focus. Even her ears are a little bit blurry. The background is completely gone. This is a very shallow depth of field.

This image, however, was shot with an aperture of f/11. I wanted the skier and the mountains in the background to be in focus. If I’d shot this at f/1.8, something would have to be blurry.

Depth of field is often the most important decision you need to make. It completely changes the look of your photos. For portraits, a wide aperture is going to look great. For group shots, landscapes, and so on, you often want a narrow aperture and all the depth of field that comes with it.

Properly Combining Aperture and Shutter Speed

To make a good exposure, you need to let a certain amount of light in. In most cases, there are a range of combinations of shutter speed and aperture that will do it. You can go with a wider aperture and a faster shutter speed, or a narrow aperture and a slower shutter speed. It’s the other “side effects” above that determine which of those is ideal.

Below, you can see four photos of the leaves shot with four different combinations of shutter speed and aperture. The exposures all look the same, but the amount of motion blur and the depth of field of each image is different. Since the leaves are moving and there’s no real background to the photo, the best photo is the one with a fast shutter speed and lower depthof field (top left).

The Third Factor: ISO

So far I’ve only been focusing on shutter speed and aperture; that’s because they’re the two most important exposure controls to understand. There is, however, a third factor that determines what each image looks like: ISO.

Rather than physically change the amount of light that falls on the camera’s sensor, ISO controls how sensitive it is to light. At lower ISOs, more light has to fall on the sensor to get the same exposure than at higher ISOs.

Light is converted to a digital signal by the sensor. If you are using a higher ISO, that signal will be amplified. The problem is, that amplifying the signal also amplifies any noise. High-ISO images often have an unpleasant noisy look.

Why didn’t we bring up ISO sooner? Well, since it’s so easy to change, some people rely on ISO way too much, using it as a cop out to control exposure without changing the shutter speed and aperture. But shutter speed and aperture are way more important creatively, and don’t have the significant downside of ISO. So, while ISO is useful, it should be your last step in the process, and only cranked up if absolutely necessary; high values are too detrimental to your images.

On most cameras, you’ll be able to use an ISO of between 100 and somewhere around 6400. However, your images will generally only look good between 100 and 1000.

In the images below, you’ll see two shots taken a few seconds apart. I’ve zoomed right in to 200% on a single leaf. The image on the left was shot at an aperture of f/22 with a shutter speed of 1/15th of a second and an ISO of 100. The image on the right also had an aperture of f/22, but I was able to use a shutter speed of 1/250th of a second because I’d increased the ISO to 1600.

You can see the effects of both shutter speed and aperture on the image. In the one where the shutter speed is slower, the image is free from noise, but has motion blur. In the one with the fast shutter speed, everything is crisp, but there’s loads of unpleasant noise.

Together, shutter speed, aperture, and ISO are known as the “exposure triangle”. They’re the three factors you control that determine how your images will look, and you’ll need to find the right balance between them for the perfect photo.

Harry GuinnessHarry Guinness is a photography expert and writer with nearly a decade of experience. His work has been published in newspapers like the New York Times and on a variety of other websites, including Lifehacker. Read Full Bio »